Update `ABSL_RETIRED_FLAG` comment to reflect `default_value` is no longer
used.

PiperOrigin-RevId: 668612700
Change-Id: I018ce756632154a9e7f2de77e6faa24296d80002

absl/flags/flag.h

diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index a8e0e93..b2c6af2 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -290,8 +290,7 @@
 // arguments unchanged (unless of course you actually want to retire the flag
 // type at this time as well).
 //
-// `default_value` is only used as a double check on the type. `explanation` is
-// unused.
+// `default_value` and `explanation` are unused.
 // TODO(rogeeff): replace RETIRED_FLAGS with FLAGS once forward declarations of
 // retired flags are cleaned up.
 #define ABSL_RETIRED_FLAG(type, name, default_value, explanation)      \